Field of the Invention
The present invention relates to a field-effect transistor, a display element, an image display device, and a system.
Description of the Related Art
Flat panel displays (FPDs), such as liquid crystal displays (LCDs), organic electroluminescent (EL) displays (OLEDs), and electronic paper, are driven by a driving circuit including a thin film transistor (TFT) using amorphous silicon or polycrystalline silicon for an active layer. FPDs have been demanded for further increase in size, higher definition, and high-speed driving performance. In order to meet these demands, there is a need for TFTs having high carrier mobility, small change in characteristics over time, and less variation between elements.
However, TFTs using amorphous silicon (a-Si) or polycrystalline silicon (in particular, low-temperature polysilicon: LTPS) for an active layer have advantages and disadvantages. It is therefore difficult to satisfy all the requirements for the TFTs at the same time.
For example, a-Si TFTs have the following disadvantages. The a-Si TFTs have insufficient mobility for driving a large liquid crystal display (LCD) at high speed and have a large shift in threshold voltage when continuously driven. LTPS-TFTs have large mobility but have large variation in threshold voltage because of a process for crystallizing an active layer by excimer laser annealing. Therefore, there is a problem that a size of mother glass for a mass-production line cannot be enlarged.
In view of the above, proposed is InGaZnO4 (a-IGZO) which can be formed into a film at room temperature and exhibits mobility equal to or higher than mobility of a-Si in an amorphous state (see K. Nomura, 5 others, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, NATURE, VOL 432, No. 25, NOVEMBER, 2004, pp. 488-492). The proposed material has been a trigger for actively studying amorphous oxide semiconductors having high mobility.
However, TFTs using the above oxide semiconductors as active layers have many problems associated with stability of characteristics of TFTs and durability to post processing. In order to organize the problems, these problems are classified into the following two: problems associated with characteristics of oxide semiconductors which are active layers; and problems associated with characteristics of source and drain electrodes.
In order to solve the above-described problems, proposed are techniques of improving an active layer (see, for example, Japanese Unexamined Patent Application Publication No. 2011-129895), techniques of disposing a buffer layer (see, for example, Japanese Patent No. 5118811 and Japanese Unexamined Patent Application Publication No. 2010-62546), techniques of improving source and drain electrodes (see, for example, Japanese Unexamined Patent Application Publication No. 2011-103402), and techniques of using specific n-type oxide semiconductors (see, for example, Japanese Unexamined Patent Application Publication No. 2013-4555).
However, there have not yet been obtained field-effect transistors that use a n-type oxide semiconductor for an active layer, realize excellent TFT characteristics with simple structures, and have excellent post-process durability and operation stability. Therefore, there is a need for such field-effect transistors.